In processes which rely on delivery of large amounts of heat energy into a furnace by combustion of a fuel, it is particularly important to achieve as high an energy-efficiency as possible. Thus it is a common practice to recover excess heat in the flue-gas, for example by using it to heat combustion air. Another way to improve efficiency is by oxy-combustion, which, by replacing air ordinarily used in combustion with a stream that is largely oxygen, avoids heating the nitrogen component of air. While heat lost to the flue gas is reduced in oxy-combustion (because the flue-gas volume is less), the amount of heat lost is still substantial, and it would be advantageous to recover that heat.
U.S. Pat. No. 5,807,418 discloses heat recovery by “co-current indirect heat exchange” of an oxidant (at least 50% O2) by the flue-gas, followed by using the partially-cooled flue-gas to pre-heat batch and/or cullet. As used by U.S. Pat. No. 5,807,418, “co-current indirect heat exchange” refers simply to a heat exchanger in which the oxidant and heat exchanger are separated by a wall, with both the oxidant and the flue gas flowing in the same direction. While a sketch is provided, details such as materials of construction of the heat exchanger are not, but for the comment that the heat exchanger is “constructed using materials and in a way that renders it compatible with and safe for handling oxygen-rich oxidants and high temperatures”. Considering the practical difficulty of constructing such a heat exchanger, this instruction is not sufficient to allow practical implementation by the skilled artisan. This scheme suffers from the apparent disadvantage that unburnt fuel in the hot combustion gases may come into contact with oxygen, either from a leak or at a regenerator, thereby posing an unacceptably high risk of catastrophic uncontrolled combustion.
US 2009/0084140 uses a scheme similar to U.S. Pat. No. 5,807,418, but with batch/cullet pre-heat in parallel with oxidant pre-heat, and with additional disclosure related to the batch/cullet heat exchanger. Again, no details on the construction of the oxidant heat exchanger are disclosed other than to say that it may be a regenerative or recuperative heat exchanger. A single stream of oxidant is sent to a heat exchanger where it is heated through heat exchange with hot combustion gases. Three streams of fuel along with three streams of pre-heated oxidant from the heat exchanger are combusted by three corresponding burners. Thus, in each case, a single stream of oxidant is sent to each heat exchanger. There is no mention of controlling the temperature of internal components of the heat exchanger. This approach, too, suffers from the disadvantage that unburnt fuel in the hot combustion gases may come into contact with oxygen, either from a leak or at a regenerator, thereby posing an unacceptably high risk of catastrophic uncontrolled combustion.
US 2009/029800 discloses a heat exchanger in which heat is exchanged between flue gases and either an oxidant or a fuel through an intermediate inert gas. Thus, a wall separates the flue gases from the inert gas and another wall separates the inert gas from the oxidant or fuel. While this reduces the chances that catastrophic uncontrolled combustion may occur, this type of heat exchanger is relatively more complicated to construct and maintain. In each case, a single stream of oxidant is sent to each heat exchanger. Additionally, there is no mention of controlling the temperature of internal components of the heat exchanger other than to say that the intermediate gas acts as a thermal buffer to dampen variations in temperature of the oxidant or fuel.
US 2010/0258263 discloses pre-heating oxidant for a furnace where, instead of using one large heat exchanger for pre-heating many oxidant streams, it proposes the use of several, smaller-dimensioned heat exchangers for a relatively small number of burners. There is no mention of controlling the temperature of internal components of the heat exchanger.
US 2011/0104625 discloses pre-heating oxidant for a furnace where a heat transfer fluid is first pre-heated through recuperative heat exchange with hot flue gases followed by pre-heating of the oxidant through heat exchange with the hot heat transfer fluid at an additional heat exchanger. In each case, a single stream of oxidant is sent to each heat exchanger. There is no mention of controlling the temperature of internal components of the heat exchanger.
US 2011/0104628 similarly discloses pre-heating oxidant for a furnace where a heat transfer fluid is first pre-heated through recuperative heat exchange with hot flue gases followed by pre-heating of the oxidant through heat exchange with the hot heat transfer fluid at an additional heat exchanger. Again, in each case a single stream of oxidant is sent to each heat exchanger. It specifically discloses that the number of heat exchangers in relation to the number of burners should be increased so that the lengths of the hot oxidant lines from the heat exchanger to the burners can be minimized to avoid thermal losses. There is no mention of controlling the temperature of internal components of the heat exchanger.
U.S. Pat. No. 6,250,916 also discloses pre-heating oxidant for a furnace where a heat transfer fluid is first pre-heated through recuperative heat exchange with hot flue gases followed by pre-heating of the oxidant through heat exchange with the hot heat transfer fluid at an additional heat exchanger. Again, in each case a single stream of oxidant is sent to each heat exchanger. There is no mention of controlling the temperature of internal components of the heat exchanger.
Many have proposed designs for performing heat exchange with a shell and tube heat exchangers where the temperature of either the tube or or shell side fluid is managed using a bypass for bypassing an amount of the fluid past the heat exchanger (i.e., U.S. Pat. Nos. 7,234,512; 4,593,757, 6,003,954; 5,615,738; and 4,991,643; US 2009/0320642; U.S. Pat. Nos. 6,302,191; 6,003,594, and5,615,738). The basic concept of these designs is illustrated in FIG. 1 where a feed flow of a tube-side fluid is divided between a main flow and a bypass flow. The main flow is heated or cooled by the shell-side fluid inside the heat exchanger HE while the bypass flow completely bypasses it. After bypass, each of the fluid flows are combined. The temperature of the combined flow is measured. In the case of a tube-side fluid being heated by a shell-side fluid, if the temperature is higher than a predetermined setpoint, a controller sends a signal to a control valve commanding it to increase the amount of the tube-side fluid that bypasses the heat exchanger. In this manner, the temperature of the combined flow may be managed to some degree. Shell side bypass can be achieved using external valves or mechanically manipulating baffles (i.e., U.S. Pat. No. 6,003,594 and U.S. Pat. No. 5,615,738)—the latter seems complex to be constructed. Other proposals (US 2009/0320642, U.S. Pat. No. 4,991,643) show methods of tube side internal bypass but they require sophisticated valves.
There have been some other proposals for shell side internal bypass, but in each of those cases, the purpose of the bypass is to improve the efficiency with better flow distribution, not for control of temperature.
The proposal illustrated in FIG. 1 does provide some advantages. Controlling the temperature of the combined flows is important, because a relatively constant process gas temperature can be achieved. Also, the temperature of the process gas can be controlled to a degree that the temperature of any process gas conveying pipes located downstream of the heat exchanger can be maintained below their material limit. For example, in the case of an oxidative process gas the downstream pipes must be kept below the material limit of those pipes in order to avoid premature or catastrophic failure of those pipes.
However, the complete bypass technique illustrated in FIG. 1 is also associated with some disadvantages. There can be a temperature overshoot inside the heat exchanger when the amount of bypass is too large. This is because when the flow rate of the bypass is increased, the flow rate of the tube-side fluid flowing through the heat exchanger is correspondingly decreased. A decreased flow rate of the tube-side fluid inside the heat exchanger will increase the heat exchange rate between the tube-side fluid and the hot shell-side fluid. As a result, although the setpoint temperature of the combined flows might not be exceeded downstream of the heat exchanger, the temperature inside the heat exchanger may reach unsatisfactorily high levels. When the tube-side fluid is reactive or oxidative, such as oxygen or oxygen-enriched air, unsatisfactorily high temperatures can cause corrosion and/or uncontrolled combustion inside the heat exchanger. This poses an unacceptably high risk of premature or catastrophic failure. This high risk is exacerbated when the reactive oxidative tube-side fluid follows a flow path having bends/turns that introduce localized high turbulence. Highly turbulent flow increases the rate at which the tube-side fluid reacts with or oxidizes the tube-side portion of the heat exchanger adjacent the highly turbulent flow.
For this reason, the solution illustrated in FIG. 1 must be operated well below the actual limit temperature at which the internal components of the heat exchanger may fail. For instance, if the maximum internal temperature ever reached within the heat exchanger is only 500° C. (which is lower than the limit temperature of 580° C.) perhaps as much as 20-30% bypass of the tube-side fluid can be achieved without fear of reaching the limit temperature 580° C. The necessity of having to operate the heat exchanger at a temperature well below the material limit temperature of the heat exchanger internal components can significantly constrain the range of safely achievable tube-side gas temperatures. In order to reach higher tube-side temperatures, the heat exchanger must be manufactured from exotic and high expensive materials.
Thus, there is a need for a method and heat exchange system that does not exhibit an unacceptably high risk of premature and potential catastrophic failure. There is also a need for a method and heat exchange system that reduces the number of heat exchangers in relation to the number of burners receiving pre-heated oxidant or fuel from the heat exchangers. There is also a need for a method and heat exchange system that controls the temperature of internal components of a heat exchanger without incurring temperature overshoots. There is also a need for a method and heat exchange system that may safely achieve a relatively wider range of process gas temperatures at a reasonable cost. There is still also a need for a method and heat exchange system that may safely achieve a relatively wider range of process gas temperatures while still allowing the heat exchanger to be manufactured with a wide variety of materials.